1. Field of the Invention
The present invention relates to an improved method for reducing the dead-periods in magnetic resonance imaging pulse sequences and to associated apparatus and, more specifically, it relates to a simplified means for reducing dead-periods by employing a calculated waveform and, most specifically, is particularly advantageous in imaging oblique planes.
2. Description of the Prior Art
The advantageous use of non-invasive and non-destructive test procedures has long been known in both medicine and industrial applications. In respect of medical uses that limit a patient's exposure to potentially damaging x-ray radiation, it has been known to accomplish this objective through the use of other non-invasive imaging procedures, such as, for example, ultrasound imaging and magnetic resonance imaging. See, for example, U. S. Pat. Nos. 4,766,381; 5,099,208 and 5,352,959.
In a general sense, magnetic resonance imaging involves providing bursts of radio frequency energy to a specimen positioned in a main magnetic field in order to induce responsive emission of magnetic radiation from the hydrogen nuclei or other nuclei. The emitted signal may be detected in such a manner as to provide information as to the intensity of the response and the spatial origin of the nuclei emitting the responsive magnetic signal. In general, the imaging may be performed in a slice or plane, or multiple planes, or three-dimensional volume with information corresponding to the responsively emitted magnetic radiation being delivered to a computer which stores the information in the form of numbers corresponding to the intensity of the signal. The computer establishes a pixel value as by employing Fourier Transformations which convert the signal amplitude as a function of time to signal amplitude as a function of frequency. The signals may be stored in the computer and may be delivered with or without enhancement to a video screen display, such as a cathode-ray tube, for example, wherein the image created by the computer output will be presented through regions of contrasting black and white which vary in intensity or color presentations which vary in hue and intensity.
The challenges involved in producing high resolution, accurate images in a timely fashion become more complex and difficult to solve when imaging oblique planes. Such oblique plane imaging is frequently employed, for example, in breath-hold magnetic resonance cardiac imaging.
Efforts to employ trapezoidal waveforms which have been employed in non-oblique planes in oblique imaging have resulted in the problems of maximum slew rate and gradient level limitation because the waveforms applied to the gradient amplifiers would not be identical to those applied in non-oblique imaging. The waveforms applied to the amplifiers are a function of the orientation of the slice being imaged. If these non-oblique imaging waveforms were applied in oblique imaging, the gradients would not be able to produce the waveforms and image artifacts and distortions result.
It has been suggested to solve this problem by avoiding application of two or more gradient waveforms simultaneously. See Bernstein et al., "Pulse Sequence Generated Oblique Magnetic Resonance Imaging: Application to Cardiac Imaging," Med. Phys., Vol. 13, pp. 648-657 (1986), Erratum: Med. Phys. 14(1):145, (1987).
It has also been suggested to decrease the slew rates and maximum allowed gradient levels in designing trapezoidal waveforms by a factor of the .sqroot.3. The problem with this approach is that it tends to cause an unnecessary increase in the minimum possible TE and TR. Another approach, which also employs trapezoidal waveforms, calculates the slew rates and maximum gradient levels depending on the orientation of the slice and the number of active gradients. See Bernstein et al., "Angle-Dependent Utilization of Gradient Hardware for Oblique MRI," J. Mag. Reson. Imag., Vol. 4, pp. 105-108 (January 1994). This last method will be referred to herein as the "scan plane optimization" method.
Another approach would be to employ gradient amplifiers and coils that allow higher gradient levels and higher slew-rates. This, of course, would involve the expense of acquiring new hardware.
In spite of the foregoing known systems, there remains, a very real and substantial need for a method of magnetic resonance imaging which is capable of producing faster, high quality magnetic resonance images on an oblique plane in reduced data acquisition time.